Feedback Based on Codebook Subset

ABSTRACT

The present invention provides for an improved application of signal strength weightings in a SDMA sectorized cellular network. The improved signal strength weightings application is conducted through the improved selection of weightings from a new codebook subset or by the selection of weightings from a larger codebook subset. In a further embodiment, an antenna beam index or bit map can be used to select the best beam(s) in a SDMA sectorized cellular network. In another embodiment, a field or factor in an uplink or downlink transmission packet can designate which directional transmission beam is best suited for the transmission or when the directional transmission beam should be activated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of

U.S. patent application Ser. No. 15/331,979, filed Oct. 24, 2016, titled“Feedback Based on Codebook Subset”, by Lai King Tee, Yi Song and NengWang, which is a continuation of

U.S. patent application Ser. No. 14/997,871, filed Jan. 18, 2016, titled“Feedback Based on Designated Subset of Codebook”, by Lai King Tee, YiSong and Neng Wang, now U.S. Pat. No. 9,479,239, which is a continuationof

U.S. patent application Ser. No. 14/561,285, filed Dec. 5, 2014, titled“Codebook Subset Selection”, by Lai King Tee, Yi Song and Neng Wang, nowU.S. Pat. No. 9,270,351, which is a continuation of

U.S. patent application Ser. No. 14/146,764, filed Jan. 3, 2014, titled“Codebook Subset Selection”, by Lai King Tee, Yi Song and Neng Wang, nowU.S. Pat. No. 8,909,156, which is a continuation of

U.S. patent application Ser. No. 13/758,446, filed Feb. 4, 2013, titled“Weighting Matrix Selection Based on Information Acquired from RemoteStation”, by Lai King Tee, Yi Song and Neng Wang, now U.S. Pat. No.8,644,764, which is a continuation of

U.S. patent application Ser. No. 12/989,749, filed Oct. 26, 2010, titled“Performance for a Multiple Antenna Beamforming Cellular Network”, byLai King Tee, Yi Song and Neng Wang, now U.S. Pat. No. 8,391,797, whichis

the U.S. National Stage of International Application No. PCT/US09/02585,filed Apr. 28, 2009, which claims the benefit of priority to

U.S. Provisional Application No. 61/048,716, filed on Apr. 29, 2008.

All of the above-identified applications are hereby incorporated byreference in their entireties as though fully and completely set forthherein.

The claims in the instant application are different than those of theparent application or other related applications. The Applicanttherefore rescinds any disclaimer of claim scope made in the parentapplication or any predecessor application in relation to the instantapplication. The Examiner is therefore advised that any such previousdisclaimer and the cited references that it was made to avoid, may needto be revisited. Further, any disclaimer made in the instant applicationshould not be read into or against the parent application or otherrelated applications.

FIELD OF THE INVENTION

A system and method for selection of codebook subset in a mobilecommunication system having multiple transmit antennas.

BACKGROUND OF THE INVENTION

There is an increasing demand on mobile wireless operators to providevoice and high-speed data services, and at the same time, theseoperators want to support more users per base station to reduce overallnetwork costs and make the services affordable to subscribers. As aresult, wireless systems that enable higher data rates and highercapacities are needed. The available spectrum for wireless services islimited, and the prior attempts to increase traffic within a fixedbandwidth have increased interference in the system and degraded signalquality.

One problem exists when prior art omni-directional antennas are used atthe base station because the transmission/reception of each user'ssignal becomes a source of interference to other users located in thesame cell location on the network, making the overall systeminterference limited. Such an omni-directional antenna is shown in FIG.1(a). In these traditional mobile cellular network systems, the basestation has no information on the position of the mobile units withinthe cell and radiates the signal in all directions within the cell inorder to provide radio coverage. This results in wasting power ontransmissions when there are no mobile units to reach, in addition tocausing interference for adjacent cells using the same frequency, socalled co-channel cells. Likewise, in reception, the antenna receivessignals coming from all directions including noise and interference.

An effective way to reduce this type of interference is to use multipleinput-multiple output (MIMO) technology that supports multiple antennasat the transmitter and receiver. For a multiple antenna broadcastchannel, such as the downlink on a cellular network, transmit/receivestrategies have been developed to maximize the downlink throughput bysplitting up the cell into multiple sectors and using sectorizedantennas to simultaneously communicate with multiple users. Suchsectorized antenna technology offers a significantly improved solutionto reduce interference levels and improve the system capacity.

The sectorized antenna system is characterized by a centralizedtransmitter (cell site/tower) that simultaneously communicates withmultiple receivers (user equipment, cell phone, etc.) that are involvedin the communication session. With this technology, each user's signalis transmitted and received by the base station only in the direction ofthat particular user. This allows the system to significantly reduce theoverall interference in the system. A sectorized antenna system, asshown in FIG. 1(b), consists of an array of antennas that directdifferent transmission/reception beams toward each user in the system ordifferent directions in the cellular network based on the user'slocation.

The radiation pattern of the base station, both in transmission andreception, is adapted to each user to obtain highest gain in thedirection of that user. By using sectorized antenna technology and byleveraging the spatial location of mobile units within the cell,communication techniques called space-division multiple access (SDMA)have been developed for enhancing performance. Space-Division MultipleAccess (SDMA) techniques essentially creates multiple, uncorrelatedspatial pipes transmitting simultaneously through beamforming and/orprecoding, by which it is able to offer superior performance in multipleaccess radio communication systems.

This method of orthogonally directing transmissions and reception ofsignals is called beamforming, and it is made possible through advancedsignal processing at the base station. In beamforming, each user'ssignal is multiplied with complex weights that adjust the magnitude andphase of the signal to and from each antenna. This causes the outputfrom the array of sectorized antennas to form a transmit/receive beam inthe desired direction and minimizes the output in other directions,which can be seen graphically in FIG. 2.

While known methods exist in the conventional multi-user multipleantenna systems that employ an orthogonal precoder to place weightingson the spatially orthogonal beamforming transmissions, the known methodsand systems are not optimized in the precoding operations, and therebyfail to optimize the performance on the network. The present inventionresolves these problems. Further, the installation of many antennas atsingle base stations can have many challenges which are resolved by thepresent invention. Since the available spectrum band will probably belimited while the requirement of data rate will continuously increase,the present invention also supports an expansion of the availablespectrum over known methods for precoding in the cellular network.

The various components on the system may be called different namesdepending on the nomenclature used on any particular networkconfiguration or communication system. For instance, “user equipment”encompasses PC's on a cabled network, as well as other types ofequipment coupled by wireless connectivity directly to the cellularnetwork as can be experienced by various makes and models of mobileterminals (“cell phones”) having various features and functionality,such as Internet access, e-mail, messaging services, and the like.

Further, the words “receiver” and “transmitter” may be referred to as“access point” (AP), “base station,” and “user” depending on whichdirection the communication is being transmitted and received. Forexample, an access point AP or a base station (eNodeB or eNB) is thetransmitter and a user is the receiver for downlink environments,whereas an access point AP or a base station (eNodeB or eNB) is thereceiver and a user is the transmitter for uplink environments. Theseterms (such as transmitter or receiver) are not meant to berestrictively defined, but could include various mobile communicationunits or transmission devices located on the network.

SUMMARY OF THE INVENTION

The present invention provides for an improved application of signalstrength weightings in a SDMA sectorized cellular network. The improvedsignal strength weightings application is conducted through the improvedselection of weightings from a new codebook subset or by the selectionof weightings from a larger codebook subset. In a further embodiment, anantenna beam index or bit map can be used to select the best beam(s) ina SDMA sectorized cellular network. In another embodiment, a field orfactor in an uplink or downlink transmission packet can designate whichdirectional transmission beam is best suited for the transmission orwhen the directional transmission beam should be activated.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention will become more readilyunderstood from the following detailed description and appended claimswhen read in conjunction with the accompanying drawings in which likenumerals represent like elements and in which:

FIG. 1 is a graphical illustration of an omni-directional antenna (a)and a sectorized antenna (b);

FIG. 2 is a graphical illustration of a weighted sectorized transmissionbeam directed to the desired user;

FIG. 3 is a graphical illustration of a multiple antenna transmissionsystem using precoding;

FIG. 4 is a codebook subset table for constant modulus;

FIG. 5 is a codebook subset table for antenna selection;

FIG. 6 is a precoding codebook subset table;

FIG. 7 is a precoding codebook subset table;

FIG. 8 is a precoding codebook subset table proposed in the presentinvention;

FIG. 9 is a precoding codebook subset table proposed in the presentinvention;

FIG. 10 is a larger precoding codebook subset table proposed in thepresent invention; and,

FIG. 11 is a precoding codebook subset table proposed in the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1(a), the overall transmission architecture 100 of anomni-directional antenna 105 that transmits radially outward equally invarious directions shown by arrows 125, 115, 135 and 140. The perimeterof the coverage area is shown by the area 120 for the transmissionarchitecture 100. Improved efficiencies have been achieved by using thesectorized antenna architecture 140 shown in FIG. 1(b).

Multiple antennas 145, 147 and 148 are shown in the architecture 140,wherein each antenna is directed toward a different region of thecellular network shown by the directional transmission 175 for coveragearea 150, transmission 190 for coverage area 157, and directionaltransmission 180 for coverage area 155. In this context, it is possiblefor system capacity to be improved by the sectorized architecture.

By weighting the various transmission signals, additional efficienciesand reduced interferences can be achieved as shown in FIG. 2 for thesectorized architecture 200. Multiple antenna elements 215, 220, 227 and230 direct transmissions (or receive transmissions) in the sectorizedantenna architecture 200. A directional antenna beam 235 is formed byscaling the signal with a set of weighting factors applied to an arrayof antenna elements, such as antenna element 230. The desired user 205is shown receiving a desired transmission 245 in the coverage area ofthe directional antenna beam 235, which is a heavily weightedtransmission meant to be directed to that user 205. An interfering user210 is shown with less weighted transmission signals 240 to reduce theinterference encountered by that user 210.

In FIG. 3, a precoding architecture 300 is shown where a data input 301is fed into the user selection component 310. The user selectioncomponent 310 sends the appropriate data through the appropriate datasignal line 315 to the precoding component 321. The appropriate data foreach user 350, 351, 352 may consist of channel encoded, interleaved,rate-matched, scrambled and/or modulated symbols. The precodingcomponent 321 provides an appropriate weighting for the signal strengthto be transmitted on the multiple antennas 320, 322 or 325. Based on thetargeted user 350, 351 and 352, the signal strength weighting of themultiple antennas to each of these targeted user will be adjusted toincrease the efficiency of the data transfer to the desired user andreduce interference with other users on the system.

The selection of specific codes to be used in the precoding component321 to provide appropriate weightings for the signal strength are shownin several tables documented in FIGS. 4-11. In FIG. 4, a constantmodulus 2-Tx codebook is shown, and in FIG. 5, an antenna selection 2-Txcodebook is shown. A codebook accepted under the TS 36.211 v8.2.0standard is shown in FIG. 6.

There are two possible configurations for the codebook selection usingthe codebooks at FIGS. 4, 5 and 6. In one configuration, the attachmentpoint (base station/antenna) may select one of the two subsets shown inFIG. 4 or 5 for use in a sector where the user is located. Theattachment point selects a subset codebook for all user equipment in thesame sector, such as using only the codebook shown in FIG. 4 or 5. Theattachment point selects the codebook subset for the user equipmentbased on some knowledge of the user equipment's channel condition. Thechannel condition information includes information regarding the userequipment's location information, the error rate for transmissions tothe user equipment, the number of re-transmissions to the userequipment, and the uplink sounding or other uplink transmissions, withthe uplink received beam-forming using a similar beam pattern as thatfor the downlink transmission.

In a second configuration, the user equipment can select the appropriatecodebook subset to be used in FIGS. 6, and the user equipment can selectbetween a total of 9 different distinct codewords for a 2-Tx twotransmission antenna system. The user equipment transmits an indicatorthat implicitly or explicitly indicates which codebook subset is chosen.The subset selection will be dictated in the second configurationthrough a higher layer activation depending on the codeword selectedfrom the codewords shown in FIG. 6, and the index of the selectedcodeword in the subset is signaled using 2 bits through the normal PMIfeedback indicator field value. To support this approach, the PMIindicator for both the downlink and uplink signaling needs 2-bits.

As an alternative, the codebook shown in FIG. 7 can be substituted forthe various codebooks shown above in FIG. 4 or 5. Instead of using thepreviously-identified codebooks in FIGS. 4-7, the present invention alsosupports the use of codebook subsets shown in FIGS. 8 and 9, either ofwhich can be used in the above configurations. That is, the codebooks inFIGS. 8 and 9 can be selected using two configurations.

In one configuration, the attachment point (base station/antenna) mayselect one of the two subsets shown in FIG. 7, and either FIG. 8 or 9for use in a sector where the user is located. The attachment pointselects a subset codebook for all user equipment in the same sector,such as using only the codebook shown in either FIG. 8 or 9. Theattachment point selects the codebook subset for the user equipmentbased on some knowledge of the user equipment's channel condition. Thechannel condition information includes information regarding the userequipment's location information, the error rate for transmissions tothe user equipment, the number of re-transmissions to the userequipment, and the uplink sounding or other uplink transmissions, withthe uplink received beam-forming using a similar beam pattern as thatfor the downlink transmission.

In a second configuration, the user equipment can select the appropriatecodebook subset to be used in either FIG. 8 or 9, and the user equipmentcan select between the different distinct codewords for a twotransmission antenna (2-Tx) system. The user equipment transmits anindicator that implicitly or explicitly indicates which codebook subsetis chosen. The subset selection will be dictated in the secondconfiguration through a higher layer activation depending on thecodeword selected from the codewords shown in FIG. 7, and either FIG. 8or 9, and the index of the selected codeword in the subset is signaledusing 2 bits through the normal PMI feedback indicator field value. Tosupport this approach, the PMI indicator for both the downlink anduplink signaling needs 2-bits.

Further, the attachment point may also use a larger codebook subsettable as shown in FIGS. 10 and 11 for use in a sector where the user islocated. The attachment point selects a codebook for all user equipmentin the same sector, such as using only the codebook shown in FIG. 10 or11. To support this approach, the original codebook with antennaselection codewords will be optimized using 3 bits, and the PMIindicator for both the downlink and uplink signaling needs 3-bits toallow the proper selection of the increased number of codewords. Theselection of the codebook subset for this configuration can also beconfigured using the Radio Resource Configuration (RRC) signaling, whichcan select the use of codebooks in FIG. 10 or 11 instead of otherdefault codebook subsets set by the system. The attachment point mayalso select the codebook subset for the user equipment based on someknowledge of the user equipment's channel condition. The channelcondition information includes information regarding the userequipment's location information, the error rate for transmissions tothe user equipment, the number of re-transmissions to the userequipment, and the uplink sounding or other uplink transmissions, withthe uplink received beam-forming using a similar beam pattern as thatfor the downlink transmission.

The application of the signal strength weightings can also be optimizedusing an antenna beam indicator. The indicator may be a field in theuplink or downlink transmission packets. The length (number of bits) forsuch an indicator will depend on the number of available antennas in thenetwork location. One bit length is sufficient for two antennaarchitectures, while 2 bits is sufficient to designate up to fourantennas. The antenna beam indicator can also be designated according toa bit map with each bit identifying one of the available beams that canbe used to communicate with the user equipment.

Based on the specific beam location, the user equipment will provide anindicator bit value or bit map value indicating which beam can providethe best coverage for that user equipment. The use of that antenna beamindicator over a specific period of time will depend on the userequipment mobility, with the indicator being valid longer for slowermoving user equipment and being valid for a shorter period of time forfaster moving user equipment. Thus, the antenna beam indication needs tobe updated with a periodicity corresponding to the changes.

The use of an antenna beam indicator is made possible through theestimation of the uplink transmission condition, such as an analysis ofthe sounding, random access, or other types of uplink transmissions fromthe user equipment. The access point may also use a direction-findingalgorithm to determine the beam index for user equipment using the SDMAprotocols. The CQI index can be used to provide selection information tothe access point, which can also analyze the signal-to-interference andnoise ratio and identification of the serving beam for the userequipment.

In systems with switching beams or opportunistic beams (e.g. OSTMA), theuser equipment provides a CQI index when it is within the coverage areaof a beam that has been switched (powered) on. Based on the time whenthe CQI is received by the access point, the beam index can beimplicitly determined because the beam pattern is known by the accesspoint.

The technology as described above allows the configuration of additionalcodebooks for UE feedback in closed-loop operations, so that a moreappropriate codebook can be used to support different antennaconfigurations, e.g. correlated, uncorrelated or cross-polarized antennasystems. To allow the support of various antenna configurations thatwould be favorable for different deployment scenarios, e.g., correlated,uncorrelated or cross-polarized antenna systems, LTE-Advanced maysupport additional codebooks to be used for UE feedback in closed-loopoperations. For backward compatibility, higher-layer (RRC) signaling canbe used to configure the use of a different codebook by some or all ofthe UEs conveniently, depending on the UE capability, e.g., Rel-8 UEs orLTE-A UEs, and the deployment configuration, e.g., correlated,uncorrelated or cross-polarized antenna systems. As the codebook isconfigurable, the larger UE-specific codebook can be configured when ahigher capacity is required in the deployed system. Otherwise, thesmaller codebook can be used to minimize UE complexity.

While the foregoing has been with reference to a particular embodimentof the invention, it will be appreciated by those skilled in the artthat changes in this embodiment may be made without departing from theprinciples and spirit of the invention, the scope of which is defined bythe appended claims.

1. A method for operating a base station, the method comprising:receiving PMI feedback from a plurality of user equipment (UE) devicesin a sector of the base station, wherein, for each of the UE devices,the PMI feedback from the UE device refers to a respective codeword fromwithin a first subset consisting of six constant modulus codewords,wherein the first subset of six constant modulus codewords is a subsetof a precoding codebook, wherein the precoding codebook consists of saidfirst subset of six constant modulus codewords and a second subset ofone or more antenna selection codewords, wherein the one or more antennaselection codewords include the codeword given by${\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}},$ wherein the first subset of six constant moduluscodewords are: ${\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\1\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- 1}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\j\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- j}\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}},{{\frac{1}{2}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}};}$ determining precoding codewords for the respective UEdevices, wherein each of the precoding codewords belongs to the firstsubset of six constant modulus codewords; for each of the UE devices,precoding respective downlink data using the respective precodingcodeword to obtain respective precoded data; and transmitting downlinksignals including the precoded data for the UE devices, wherein, for alluser equipment devices in said sector that are configured for PMIfeedback in a closed-loop 2-TX antenna configuration operation, saidcodeword referred to by said PMI feedback and said determined precodingcodewords are from within said first subset.
 2. The method of claim 1,wherein the PMI feedback from each of the UE devices is a 2-bit index.3. The method of claim 1, wherein the first subset is configured via RRCsignaling.
 4. The method of claim 1, wherein the precoding codebookincludes six rows and two columns, wherein a first of the columnsincludes the following codewords for single-layer transmission:$\begin{bmatrix}1 \\0\end{bmatrix},\begin{bmatrix}0 \\1\end{bmatrix},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\1\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- 1}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\j\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- j}\end{bmatrix}},$ wherein a second of the columns includes the followingcodewords for two-layer transmission:${\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}},{{\frac{1}{2}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}}.}$
 5. The method of claim 1, further comprising:acquiring information about the channel condition for the UE device,which includes location information and error indicators the UE device;and selecting the first or second subset for the UE device from one ormore designated codebook subsets based on the channel conditioninformation obtained about the UE device.
 6. The method of claim 1,wherein the transmitted downlink signals are weighted using weightingfactors of precoding codewords making it less likely that transmittedsignals sent to a plurality of UE devices will interfere with eachother.
 7. A base station, comprising: digital circuitry configured to:receive PMI feedback from a plurality of user equipment (UE) devices ina sector of the base station, wherein, for each of the UE devices, thePMI feedback from the UE device refers to a respective codeword fromwithin a first subset consisting of six constant modulus codewords,wherein the first subset of six constant modulus codewords is a subsetof a precoding codebook, wherein the precoding codebook consists of saidfirst subset of six constant modulus codewords and a second subset ofone or more antenna selection codewords, wherein the one or more antennaselection codewords include the codeword given by${\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}},$ wherein the first subset of six constant moduluscodewords are: ${\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\1\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- 1}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\j\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- j}\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}},{{\frac{1}{2}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}};}$ determine precoding codewords for the respective UEdevices, wherein each of the precoding codewords belongs to the firstsubset of six constant modulus codewords; precode respective downlinkdata, for each of the UE devices, using the respective precodingcodeword to obtain respective precoded data; and transmit downlinksignals including the precoded data for the UE devices, wherein, for alluser equipment devices in said sector that are configured for PMIfeedback in a closed-loop 2-TX antenna configuration operation, saidcodeword referred to by said PMI feedback and said determined precodingcodewords are from within said first subset.
 8. The base station ofclaim 7, wherein the PMI feedback from each of the UE devices is a 2-bitindex.
 9. The base station of claim 7, wherein the first subset isconfigured via RRC signaling.
 10. The base station of claim 7, whereinthe precoding codebook includes six rows and two columns, wherein afirst of the columns includes the following codewords for single-layertransmission: $\begin{bmatrix}1 \\0\end{bmatrix},\begin{bmatrix}0 \\1\end{bmatrix},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\1\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- 1}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\j\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- j}\end{bmatrix}},$ wherein a second of the columns includes the followingcodewords for two-layer transmission:${\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}},{{\frac{1}{2}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}}.}$
 11. The base station of claim 7, wherein the digitalcircuitry is further configured to: acquire information about thechannel condition for the UE device, which includes location informationand error indicators the UE device; and select the first or secondsubset for the UE device from one or more designated codebook subsetsbased on the channel condition information obtained about the UE device.12. The base station of claim 7, wherein the transmitted downlinksignals are weighted using weighting factors of precoding codewordsmaking it less likely that transmitted signals sent to a plurality of UEdevices will interfere with each other.
 13. A non-transitorycomputer-readable memory medium storing software instructions that, whenexecuted by a processor of a base station, cause the base station to:receive PMI feedback from a plurality of user equipment (UE) devices ina sector of the base station, wherein, for each of the UE devices, thePMI feedback from the UE device refers to a respective codeword fromwithin a first subset consisting of six constant modulus codewords,wherein the first subset of six constant modulus codewords is a subsetof a precoding codebook, wherein the precoding codebook consists of saidfirst subset of six constant modulus codewords and a second subset ofone or more antenna selection codewords, wherein the one or more antennaselection codewords include the codeword given by${\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}},$ wherein the first subset of six constant moduluscodewords are: ${\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\1\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- 1}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\j\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- j}\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}},{{\frac{1}{2}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}};}$ determine precoding codewords for the respective UEdevices, wherein each of the precoding codewords belongs to the firstsubset of six constant modulus codewords; precode respective downlinkdata, for each of the UE devices, using the respective precodingcodeword to obtain respective precoded data; and transmit downlinksignals including the precoded data for the UE devices, wherein, for alluser equipment devices in said sector that are configured for PMIfeedback in a closed-loop 2-TX antenna configuration operation, saidcodeword referred to by said PMI feedback and said determined precodingcodewords are from within said first subset.
 14. The non-transitorycomputer-readable memory medium of claim 13, wherein the PMI feedbackfrom each of the UE devices is a 2-bit index.
 15. The non-transitorycomputer-readable memory medium of claim 13, wherein the first subset isconfigured via RRC signaling.
 16. The non-transitory computer-readablememory medium of claim 13, wherein the precoding codebook includes sixrows and two columns, wherein a first of the columns includes thefollowing codewords for single-layer transmission: $\begin{bmatrix}1 \\0\end{bmatrix},\begin{bmatrix}0 \\1\end{bmatrix},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\1\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- 1}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\j\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- j}\end{bmatrix}},$ wherein a second of the columns includes the followingcodewords for two-layer transmission:${\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}},{{\frac{1}{2}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}}.}$
 17. The non-transitory computer-readable memory mediumof claim 13, wherein the software instructions, when executed by theprocessor, further cause the base station to: acquire information aboutthe channel condition for the UE device, which includes locationinformation and error indicators the UE device; and select the first orsecond subset for the UE device from one or more designated codebooksubsets based on the channel condition information obtained about the UEdevice.
 18. The non-transitory computer-readable memory medium of claim13, wherein the transmitted downlink signals are weighted usingweighting factors of precoding codewords making it less likely thattransmitted signals sent to a plurality of UE devices will interferewith each other.